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The effect of normal stiffness on the shear resistance of rock.

This paper presents the theory and design of a shear machine whose normal stiffness could be varied by four orders of magnitude. Experimental techniques, testing procedures, and special equipment designed by the Bureau of Mines for determining the effect of normal stiffness on the shear stress-normal displacement behavior of intact and prepared joint surfaces of both hard (granite) and soft (sandstone) rock under constant shear displacement rate are described. Experimental results are presented which show that the effect of increasing the normal stiffness on either intact rock or fracture surfaces is to provide increased normal confinement. Increased normal confinement results in an increased shear strength rock. Engineering significance of this result is discussed.